The present invention relates generally to furnaces or combustion systems and particularly to a control device for shutting down a combustion system when pressure in the venting system exceeds atmospheric pressure. Specifically, the present invention relates to a device for bringing the products of combustion within the flue pipe of a combustion system into contact with a thermal switch when abnormal conditions in the flue or venting system cause an increase in pressure above atmospheric pressure. The thermal switch is electrically connected to a fuel supply system of the combustion system and shuts down the system when the hot products of combustion from the flue pipe contact the thermal switch indicating that a pressure greater than atmospheric pressure is present in the flue pipe.
A combustion system, such as a furnace, normally contains a combustion chamber wherein fuel such as natural gas or oil is burned creating hot gaseous products of combustion, a heat exchanger wherein the heat from the products of combustion is transferred to another medium such as air or water, and a flue or vent system wherein the gases, now referred to as flue gases, pass from the heat exchanger to atmosphere at a remote location. In a natural draft combustion system the products of combustion pass over a heat exchanger and are collected at a draft hood or other such device. The draft hood is connected to a flue pipe whereby the flue gases are drawn into the flue pipe from the draft hood due to the natural draft of the vent system. In a forced draft system a fan is located upstream of the combustion chamber and forces air into the combustion chamber and the products of combustion through the heat exchanger into the flue. In an induced draft system a fan is located between the heat exchanger and the flue and draws air into the combustion chamber and draws the products of combustion through the heat exchanger into the flue.
In the natural draft system the flue or vent system usually contains a means, such as a draft hood or a draft diverter, for preventing pressure buildup in the flue if the flue is blocked. Draft hoods consist of an opening in the flue system covered by a hood. The opening allows the flue gas to vent to the atmosphere surrounding the furnace when pressure above atmospheric builds up in the flue. When the flue is functioning normally the reduced pressure in the flue draws air through the opening into the draft hood and up the flue.
In addition to providing relief for pressure in the flue, the draft hood provides dilution of the products of combustion. That is, the temperature and humidity of the products of combustion are reduced by mixing with air taken in through the draft hood opening. The air drawn into the flue through the draft hood relief opening is air which surrounds the combustion system and, in most instances, has been heated to room temperature. This, in effect, is a heat loss. That is, energy in the form of gas or oil has been used to heat the air which is subsequently drawn into the flue and expelled to the atmosphere. Reduction or elimination of this dilution air reduces the heat loss and improves the system efficiency.
One method for eliminating the heat loss of a draft hood while providing the safety features of the hood is to place a pressure responsive device in communication with the gases flowing through the combustion system so that the system is shut down when the device senses a preselected positive pressure. For example, U.S. Pat. No. 2,640,447 to Blum discloses a safety system for use in an incinerator whereby the pressure within the fire box of the incinerator is sensed and a switch responsive to the sensed pressure operates to shut down the system if the pressure exceeds a given value. Pressure sensitive devices are very sensitive and fast acting. This type of response is undesirable since pressure variations within the flue are quite common due to varying temperatures of operation, external wind conditions at the flue pipe outlet, natural down drafts and other such phenomena. Therefore, pressure sensitive devices are designed to respond to a relatively high positive pressure, that is, to a pressure substantially above atmospheric. Also, these pressure devices are not designed to provide protective functions for a combustion system when the products of combustion from this first combustion system are discharged through an outlet vent also used as the outlet vent for another combustion system. Connecting the vent system of a second combustion system to a common outlet vent provides a passage whereby the products of combustion from the first combustion system can escape when there is a blockage in the common outlet vent thereby preventing the buildup of pressure within the vent system of the first combustion system. This pressure buildup is essential to the proper operation of a pressure responsive device.
It is desirable to have a device which will not shut down a combustion system in response to normal fluctuations in pressure but will detect a small pressure increase above atmospheric pressure and shut down a combustion system in response thereto.